1. Field of the Invention
The present invention relates to a current mode DC/DC converter provided with a current mode control circuit, serving as a feedback circuit for stabilizing an output voltage, which detects a current flowing through a choke coil and controls switching operation of a switching element according to a result of comparison between a detection signal due to the current and a reference signal.
2. Description of the Related Art
Generally, a DC/DC converter, specifically a current mode DC/DC converter with a current mode control circuit is one where a current flowing through a choke coil at an output side is detected and a DC output voltage supplied to a load is stabilized by controlling switching operation of a switching element according to a result of comparison between a detection signal due to the current and a reference signal.
FIG. 3 shows a circuit diagram displaying one example of such a current mode DC/DC converter. In the figure, reference symbol E denotes a DC source supplying an input voltage Vi and a series circuit of a switching element Q1 and a free wheeling diode D1 is coupled across the DC source E, while a series circuit of a choke coil L1 and a smoothing capacitor C1 is coupled across the free wheeling diode D1 and a DC output voltage Vo generated in the smoothing capacitor C1 according to switching operation of the switching element Q1 is supplied to a load resister RL, that is a load, coupled between output terminals +V and xe2x88x92V. A current mode control circuit 51, serving as a feedback circuit for stabilizing an output voltage Vo, is coupled, which is provided with; voltage divider resisters R1, R2, for detecting the output voltage, coupled between the terminals +V and xe2x88x92V, an error amplifier A1 for amplifying an error between an output voltage detection signal output from a junction point of the voltage divider resisters R1, R2 and a reference voltage Vref, a current detector 11 for detecting the current I2 flowing through the choke coil L1, a current/voltage converter 21 for converting a detection current from the current detector 11 to a voltage, a comparator for outputting a reset pulse to turn off said switching element Q1 when a voltage V1 of a coil current detection signal supplied from the current/voltage converter 21 exceeds a voltage V3 of an error signal serving as a reference signal output from an error amplifier A1, and a RS flip-flop circuit 26 for turning on the switching element Q1 by a set pulse with cycle T output from an oscillator 25 and for turning off the switching element Q1 by a reset pulse from the comparator A2.
In the circuit in FIG. 3 abovementioned, when the switching element Q1 turns on by the set pulse from the oscillator 25, the free wheeling diode D1 turns off so that an input voltage Vi is applied to the series circuit of the choke coil L1 and the smoothing capacitor C1 and so a coil current I2 increases linearly with the lapse of time. As a result, when the coil current I2 becomes greater than a current consumed by the load resister RL, that is, a load current Io, the smoothing capacitor C1 is charged so that an output voltage Vo across the smoothing capacitor C1, in turn, across the load resister RL increases. On the other hand, in the current mode control circuit 51, a voltage detection signal yielded by dividing the output voltage Vo with the voltage divider resisters R1, R2 is compared with the reference voltage Vref by the error amplifier A1 so that an error signal obtained by amplifying the magnitude of the error is fed to one side of input terminals of the comparator A2. Apart from this, the coil current I2 flowing through the choke coil L1 is detected by the current detector 11 so that the coil current detection signal corresponding to the coil current I2 is fed from the current/voltage converter 21 to the other input terminal of the comparator A2. Then, the comparator A2 compares the voltage V1 of the coil current detection signal with the voltage V3 of the error signal and when the voltage V1 of the coil current detection signal exceeds the voltage V3 of the error signal, a reset pulse is output from the comparator A2 so that a voltage level of the output terminal is changed from a H (High) level to a L (Low) level to turn off the switching element Q1.
When the switching element Q1 turns off, the free wheeling diode D1 turns on so that energy already stored in the choke coil L1 is discharged. As a result, the coil current I2 of the choke coil L1 decreases linearly with the lapse of time and when the coil current I2 becomes less than the load current Io, electric charges are supplied from the capacitor C1 to the load resister RL so that the output voltage Vo decreases. After the lapse of one cycle, the set pulse is generated from the oscillator 25 so that the switching element Q1 turns on again and the coil current I2 and the output voltage Vo begin to increase also again.
Thus, the output voltage Vo changes with ripples by switching the switching element Q1. However, an amplitude of the variation is almost negligible in comparison with the magnitude of the output voltage Vo and therefore the output voltage Vo can be regarded as substantially stable at a certain value. Further, at the time of ON state of the switching element Q1, when the coil current I2 of the choke coil L1 increases, one fourth of a cycle later, the output voltage Vo also begins to increase and at the time of OFF state of the switching element Q1, when the coil current I2 of the choke coil L1 decreases, one fourth of a cycle later, the output voltage Vo also begins to decrease. In other word, the coil current I2 and the error signal from the output terminal of the error amplifier A1 are mutually in proportional relationship.
FIG. 4 shows each of waveforms of, the load current Io at the time of steady state, the charge and discharge current I1 of the capacitor C1 and the coil current I2, in the circuit of abovementioned FIG. 3. As described above, at the time of ON state of the switching element Q1, the coil current I2 increases linearly and when the coil current I2 becomes greater than the load current Io, a flowing direction of the charge and discharge current I1 of the capacitor 1 is converted from that for discharge to that for charge. On the other hand, when the switching element Q1 turns off, the coil current I2 decreases linearly and when the coil current I2 becomes less than the load current Io, the flowing direction of the charge and discharge current I1 of the capacitor C1 is converted from that for charge to that for discharge. At steady state, according to switching operation of the switching element Q1, the charge and discharge current I1 and the coil current I2 change with ripples. (see xcex94I1, xcex94I2, in FIG. 4).
Now, in the DC/DC converter with abovementioned current mode control circuit 51, due to delay of response or the like at the time of an abrupt change of a load current in control systems comprising the error amplifier A1 and the comparator A2, there occurs a problem that stability of the output voltage Vo is lost so that the output voltage Vo changes in a great degree. Specifically, as shown in FIG. 5, if the output current Io increases abruptly, for example, at the time to, in the first place, electric charges corresponding to the abrupt increase are supplied to the load resister RL from the smoothing capacitor C1 and next, the coil current I2, in turn, also the voltage V1 of the coil current detection signal increases gradually. However, the current mode control circuit 51, due to delay of response within the circuit, cannot immediately feeds a pulse drive signal, that is required to keep the output voltage Vo constant, to the switching element Q1 so that the output voltage Vo decreases in a great degree immediately after an abrupt change in the load current Io. (see the variation xcex94Vo in FIG. 5).
Further, in the aforementioned current mode control circuit 51, a phase compensation circuit (not shown in the figures) for improving a frequency characteristic is provided with the error amplifier A1 and the comparator A2 in order to secure stability at steady state. However, since the current mode control circuit 51 is so composed that a feed back circuit itself detects the coil current I2 and the output voltage Vo, that are output, to compensate those current and voltage, necessarily the output voltage Vo is changeable if there occurs delay of response within the current mode control circuit 51.
The present invention is achieved in view of the abovementioned problem and it is an object of the present invention to provide a current mode DC/DC converter wherein an output voltage changes in not a great degree even at the time of an abrupt change in a load current.
A current mode DC/DC converter of the present invention provided with a current mode control circuit, serving as a feedback circuit for stabilizing an output voltage supplied to a load, which detects a coil current flowing through a choke coil and controls switching operation of a switching element according to a result of comparison between a detection signal of the coil current and an error signal of the output voltage, serving as a reference signal, wherein a feed forward circuit is provided which detects a variation in the load current flowing through the aforementioned load, adding the variation to the detection signal of the coil current.
In this case, since the load current is almost unchangeable at steady state, no variation in the load current is detected by the feed forward circuit so that the current mode control circuit controls, in the same way as does a conventional power converter, switching operation of the switching element according to a result of comparison between a detection signal of the coil current flowing through the choke coil and an error signal of the output voltage serving a reference signal. Accordingly, a characteristic of the current mode control at steady state is the same as that of a conventional power converter.
On the other hand, when the load current changes abruptly on account of some cause or other, the feed forward circuit detects the variation in the load current at the time to add the variation to the detection signal of the coil current. The current mode control circuit compares the signal that is the sum of a signal obtained by adding the variation in the load current to the detection signal of the coil current and the error signal of the output voltage serving as a reference signal to control switching operation of the switching element by the result of the comparison. As a result, a switching pulse by which the coil current can change quickly in response to the abrupt change in the load current can be fed from the current mode control circuit to the switching element so that the variation in the output voltage can be reduced.